avast can detect 1643 variants of the agent trojan and win32:Agent-HT is one of them (see image). What detected this trojan if you say avast didn't ?
However, there is no standard naming convention, so what another anti-virus program is calling agent.HT might not be one and the same.
What avast! version and VPS file (virus database) number, e.g. 0630-2 (see about avast!) ?